This is similar to the "Gaunilo's island" objection. It isn't really as sound as an objection as you might guess, IMO. Pink Unicorns are a limited idea, where as the ontological argument, according to some, appeal to every greatness; that no greater thing can exist. Pink and Unicorn are limited ideas, greater things can be conceived. While I don't hold to the ontological argument either, this isn't a truly sound objection, IMO.

The ontological argument is wrong on so many levels. Jefferson said the best response to unintelligible propositions is ridicule. This is it.

If all things that actually exist were ranked by "greatness" on some single well-defined continuum, then yes, there would of course have to be a maximum. "God exists in the understanding..." is just a sleight of hand that adds nothing to the argument but aids in conflating that narrow definition with the usual meaning of the word god. Also, they do not bother to define "greatness", or even show that it is logically possible to do so in a consistent way across an infinite number of possible types of measurement.

Depends on who's version you use exactly, it has had many different installments. Personally, my favorite are Spinoza, Leibniz and Schopenhauer. Schopenhauer's being the most interesting to me, as it tied in with Hinduism and didn't even intend to. One thing every single one of the arguments fails, however, is to point any any particular God, especially Spinoza. So even if it were true, it is unintelligible to choose a particular God based on its logical assertions, and thus, left back at square one of agnosticism.